Localisation systems are known such as radar that permit objects to be detected, measured and localised by sending electromagnetic waves.
The radar system sends bursts of electromagnetic waves by means of an antenna and is capable of picking up, either by the same antenna or by an additional antenna, the echo or reflection of the waves caused by the presence of the objects.
As is now known, with a synthetic aperture radar it is possible to generate images of terrestrial surfaces in which, basically, the position of one point is a function of the distance between the platform on which the radar is mounted (for example, a satellite) and said point situated on the ground.
The information provided by a radar is useful for measuring movements of the soil or of buildings by means of techniques called radar interferometry correlation. In order to be able to study with precision the movement of a particular point of an image, it is essential that said measurement point be a reflective point that remains permanent over time, that is, one that does not change with time. Thus if, for example, said point is the surface of the roof of a house, that house cannot be altered (demolished, or changed by adding a further story).
Often, however, it is necessary to study earth movements in zones that do not remain stable over time, such as wooded zones, snow-covered zones or sea zones (petroleum extraction zones), in which it is difficult to find features or structures useful to serve as stable reflecting points. In order to be able to have measuring points in such zones it is necessary to have artificial reflecting points such as the so-called passive reflectors (trihedral, dihedral or specular “corner reflectors”) and the so-called active reflectors (“active transponder”).
Corner reflectors are passive electromagnetic wave receiving and re-emitting devices that comprise metallic trihedrals that act as mirrors to the waves coming from the radar. Such corner reflectors present the disadvantage that they must be situated in zones where they cannot be covered by vegetation or snow, since they need a sight-line on the radar both for receiving the signal and for reflecting it.
The classic active devices for receiving and re-emitting electromagnetic waves have a receiving antenna that receives the signal coming from the radar and, usually, also an additional emitting antenna that resends said signal. Said systems also present the disadvantage that they need to have a sight-line on the radar in order to receive the signal and allow it to be re-emitted (it should be taken into account that the signal that comes from a radar situated on a satellite cannot be amplified to improve its reception, since said satellite is at an altitude of 800 km).
In any of the devices cited, reception and re-emission of the electromagnetic signal from a radar is impossible in zones close to the poles or subject to high snowfall, since in those zones the receiving and emitting points are always covered in snow. And with the devices cited, in order to ensure reception and re-emission of the signal in wooded zones it is essential to fell trees. Thus, it is not economically or environmentally viable in such zones to calculate earth movements by processing radar images (interferometry).